Groundwater level data from monitoring wells or boreholes are used for various purposes. For example, groundwater level data is used to determine magnitude and direction of hydraulic gradient at underground storage tanks sites, remedial investigation sites as required by environmental laws, and other sites effected by local and federal regulations. Changes in atmospheric pressure (barometric pressure) cause water levels to rise and fall within the wells. Variations in groundwater levels due to barometric pressure effects have the potential to give false readings. This can result in miscalculations of various items such as hydraulic gradients and flow directions, points of exposure, aquifer properties, and time to exposure from contaminated sites. The term "well" as used herein and in the appended claims, is intended to also encompass boreholes, such as boreholes used with tensiometers.
The effects of barometric fluctuations on water tables are well documented. Barometric pressure changes can cause changes of up to one foot in measured water level versus actual water level. Barometric pressure fluctuations in the atmosphere can significantly impact water table levels within wells.
Increases in barometric pressure cause declines in water levels and vice versa. The mechanisms in causing these effects are: (1) mechanical loading of the aquifer due to the surface load; (2) pressurization at the water surface of the open well due to the air load; (3) flow of the air between the earth's surface and the water table; (4) flow of groundwater between the water table and the aquifer; and (5) flow of groundwater between the aquifer and well.
A confined aquifer is one in which clay, or a confining bed of some other material, impedes upward movement of water. Water underneath the confining bed may be under pressure. In contrast, in an unconfined aquifer, water can rise relatively freely in the geologic material.
Changes in barometric pressure effect changes in water level in unconfined aquifers. The reason why changes in barometric pressure effect changes in water level in unconfined aquifers is as follows. The finite permeability of the unsaturated zone causes a lag in the transfer of the barometric fluctuations to the water table. Because the fluctuations are immediately transferred to the water table in a well, a pressure imbalance occurs between water in the well and water in the aquifer. This pressure imbalance produces the water level fluctuations in the well. After a step change in barometric pressure, only a portion of the change reaches the water table through the unsaturated zone. The difference between the barometric pressure change and the pressure transferred to the water table results in the change in the water table. In time, the water level in the well recovers to the level in the aquifer. Changes in barometric pressure can effect water level measurements in either confined or unconfined wells.
Several numerical solutions to adjust for effects of barometric pressure changes have been proposed. The numerical solutions require knowledge of the soil air diffusivity between land surface above the well and the water table. The diffusivity changes with soil water content which, in turn, changes over time. This error in water level measurements effects the determination of direction of groundwater flow, and the calculated rate of water travel. This error also makes it difficult to determine if water levels have changed, and can affect results of pumping tests so severely that resulting data cannot be properly analyzed.
Many of the prior art methods used to account for barometric effects on the water table use a concept known as barometric efficiency. Barometric efficiency is defined as the fraction of the change in barometric pressure that is instantaneously transmitted to the liquid in the aquifer. The barometric efficiency is calculated as the ratio of the change in the water level in a well compared to the change in atmospheric pressure.
One prior art method of determining average barometric efficiency for an aquifer comprises plotting a sum of incremental changes in the water table versus a sum of incremental changes in barometric pressure, following a number of rules. This method assumes that a single number can be used as an estimate for the barometric efficiency for the entire aquifer. However, barometric efficiency has been found to be related to the frequency of a barometric pressure signal.
Another method comprises performing a frequency domain analysis to correct water table signals in confined and unconfined aquifers to account for the fluctuations due to barometric pressure. A best fit method of barometric efficiency and sine wave frequencies is employed to correct data. The transfer function is assumed for the observed frequency response, the function is multiplied by Fourier transform of the atmospheric record, the result is inverted into the time domain, and a water level time series is subtracted. Assumptions and approximations are used in these methods. Therefore, only approximate water table values can be found. Another prior art method uses a convolution in the time domain as an alternative to the frequency domain analysis to remove barometric effects from measurements in confined aquifers. The time domain solution is derived from an inverse Fourier transform of frequency response function.
The numerical solutions are inadequate because there is a lag time between the pressure change of the atmosphere and the pressure in the soil column immediately above the water table.